Obesity and diabetes have reached epidemic levels in the United States. Diabetes and its associated complications are becoming one of the leading causes of morbidity in this country. Although several hypotheses link obesity and insulin resistance, the role of circadian rhythm in the pathogenesis of diabetes has not been well addressed. Interestingly, there is a clear circadian variation in fasting glucose, insulin sensitivity and glucose tolerance, which is severely dampened in the condition of obesity and diabetes. The orphan nuclear receptor Rev-erb( emerged as a core circadian gene. By repressing gene expression of another clock gene, Bma1, Rev-erb( functions as a negative regulator of circadian rhythm. We showed GSK3(-dependent phosphorylation is required for maintaining the protein stability of Rev-erb( and is crucial for synchronizing the circadian oscillation of Bma1 gene in vitro. To extend our knowledge on GSK3(-dependent regulation of Rev-erb(, we propose two specific aims in the first phase of this application (K99 phase), including: Aim 1. Elucidate the signal pathways upstream of GSK3( mediating Rev-erb( degradation in hepatocytes;and Aim 2. Determine in vivo roles of Rev-erb( phosphorylation by GSK3( in circadian rhythm and metabolism. A mouse model expressing Rev-erb( mutant which mimics GSK3( phosphorylation in liver will be created to address these questions. This will be the first analysis to determine the role of Rev-erb( as a clock protein in liver circadian rhythm and glucose metabolism. This study will be carried out in the Penn Diabetes Center at University of Pennsylvania under the supervision of Dr. Mitchell Lazar. The center has expertise in diabetes and obesity research, and is an outstanding environment in which to conduct the proposed project. Meanwhile, the project will provide superb training for the principle investigator, Dr. Lei Yin, to develop an academic career in the field of diabetes and obesity. The role of the ubiquitin-proteasome pathway (UPP) has not been well-studied in the process of metabolism and energy homeostasis. We recently discovered that Cullin 4A-based E3 ligase regulates protein stability of Rev-erb( as well as gene expression important for gluconeogenesis, suggesting Cullin 4A is a novel modulator functioning in both circadian rhythm and metabolism. Therefore, the specific aim 3 proposed during the R00 phase will be focused on unraveling roles of the Cullin 4A E3 ubiquitin ligase in regulating circadian rhythm, insulin signaling and glucose metabolism. We expect this work will provide a panel of evidences for the functional importance of the ubiquitin-proteasome system in the regulation of metabolism. In addition, this work may shed the new light on identifying new therapeutic targets for treating insulin resistance and diabetes.